feat: support linux auto-update

Currently the self update functionality breaks if ran on Ubuntu because it has a hard dependency on the mac-only tool launchctl:

    for cmd in ("git", "bash", "launchctl", "curl"):
        if shutil.which(cmd) is None:
            missing.append(cmd)
    if missing:
        msg = f"Missing required commands: {', '.join(missing)}"
        logger.error(msg)
        _write_update_status("error", msg)
        return

One solution is to refactor into a shared script with an adapter. I started such a solution that i am using locally but I held from creating a PR before discussing as an issue as instructed in CONTRIBUTING.md